1. Field of the Invention
The present invention relates to a differential for an automobile, and, more particularly, to an automobile differential whose locking force is variably controlled according to driving conditions.
2. Description of Related Art
Typically, automobiles are provided with differentials which drive both drive axles at same time, but allow them to turn at different speeds when negotiating turns. When one of the drive wheels gets caught in an irregular part of a road having a considerably low friction coefficient, such as mud in the road, such an automobile differential delivers the engine torque mostly to the one drive wheel, and consequently, causes slippage of the one drive wheel with respect to the road. Such slippage precludes the automobile from the ability to get out of the mud. In order to prevent the automobile from being so precluded in such a situation, a number of differentials of this sort are equipped with a differential lockup feature. When the rate of slippage of either one of the drive wheels or the speed difference between the drive wheels exceeds a predetermined critical lockup level, the differential is locked so as to be prevented from producing differential action.
It has been known to vary the critical lockup level of slippage rate. Establishing a higher critical lockup level of slippage rate allows the differential to produce sufficient differential action to improve the ability of turning or cornering on a normal road. On the other hand, when a lower critical lockup level of slippage rate is established, the differential allows only a slight differential action, while providing a reliable driving performance during traveling on a slippery road, such as a road with snow. Such a lockup type of differential is known from, for instance, Japanese Unexamined Patent Publication No. 59-11922.
In recent years, for the purpose of improving driving performance during turning or cornering, differentials are equipped with a variable differential action control feature so that differential action is variable in accordance with input torque to the differential. With this type of differential, as the input torque to the differential increases, a restrictive force against differential action is intensified more strongly. This provides an increase in limit speed during turning, so as to improve the responsiveness of acceleration even during turning or cornering.
When a quick acceleration is made while the vehicle is turning with the differential restricted in differential action, what is called "pushing under" is, in some cases, experienced depending upon steering angles. This behavior of pushing under is caused because the differential, when experiencing an increase in input torque resulting from quick acceleration, delivers the input torque mainly to the inner drive wheel which is lower in speed than the outer drive wheel, and consequently, provides a cornering force to the inner drive wheel stronger than to the outer drive wheel. The vehicle, when negotiating a corner, experiences a tendency to under-steer and turns less sharply than the driver intends. On the other hand, when engine braking is applied, resulting from a quick deceleration while the vehicle is turning with the differential restricted in differential action, the vehicle experiences so-called "over engine braking" which is caused because a reverse torque, which acts on the vehicle to brake it, is provided by engine braking mainly to the inner drive wheel which is lower in speed than the outer drive wheel, and consequently, provides a braking force stronger to the inner drive wheel than to the outer drive wheel. This over engine braking causes a tendency to oversteer, and the vehicle turns more sharply than the driver intends when the vehicle is negotiating a corner.
These tendencies are experienced more remarkably by, in particular, front-engine rear-drive vehicles even experienced to a certain extent by other types of vehicles. In any type of vehicle, the tendency of under pushing and over engine braking causes deterioration of the driving performance of vehicle during turning.